1. Field of the Invention
The invention pertains generally to the field of electronic fuel management systems for internal combustion engines and is more particularly directed to an acceleration enrichment feature during operator induced transients for closed loop fuel management systems.
2. Prior Art
There has been recognized in the electronic fuel injection art the need for fuel enrichment during certain transient conditions. Of the most important transients are those that are operator or driver induced and commonly termed accelerations or decelerations. The driveability of the automobile will be detrimentally effected if the fuel management system does not provide the right air/fuel mixture during these conditions. Electronic control units for electronic fuel injection systems today normally have auxiliary circuits of various types for enriching the fuel mixture during acceleration and decreasing or terminating the enrichment during decelerations.
Generally, the auxiliary circuits have a sensing means for determining that enrichment is necessary and for calculating an amount of enrichment based on the change or rate of change in some parameter such as manifold pressure, throttle angle, RPM, etc. These parameters or variables and combinations thereof provide a direct method of sensing the transient conditions and their magnitudes are substantially proportional to the enrichment needed.
The auxiliary circuits then commonly lengthen or provide additional acceleration enrichment (AE) pulses to the base fuel pulse produced by the fuel management system. According to this operation, the main fuel management system sets a desired air/fuel ratio that is correct for nontransient conditions and the auxiliary circuits provide the enrichment needed for the proper air/fuel mixture during transient condition.
While this theory of operation is correct in the macro sense in that many main fuel management systems do provide a desired average air/fuel ratio, the theory breaks down in the micro sense for closed loop systems. These systems provide means for correction toward the desired air/fuel ratio or operating point and are continually hunting for that value. It is generally understood that with modern closed loop integral control, most of the system operation is not exactly at the desired ratio. At any instant, the air/fuel ratio maybe more or less than the desired value and it is only the summation or average of the instantaneous points that produce a desired fuel ratio.
Therefore, it is seen that if the auxiliary circuit provides acceleration enrichment based only on the transient variables when injecting additional or lengthening the basic pulse width during air/fuel ratios that are rich the combination will be excessively rich and conversely when injecting pulses or modifying pulse width during lean excursions the combination will not be rich enough. These differences between the ideal response and the actual response of the air/fuel ratio will tend to average out for very long accelerations but at the expense of smooth and instantaneous accelerations.
Moreover, when the actual operating conditions of the engine are not accounted for, particularly when in some systems the closed loop control is cut out during transient conditions, the system may operate far from the desired operating point causing emissions to rise considerably or the before-mentioned driveability problems. The closed loop integral controllers will return to the desired operating point at some integration rate after cessation of the transient but the further the transient has moved the system from the desired point, the longer it will take to return it. For example, with a system operating rich with the integral controller still heading in the rich direction, an acceleration enrichment transient will shift the operation substantially from that desired.
Thus, a better method can be devised where the acceleration enrichment is not only a function of the parameters that are directly changed because of operator induced transient conditions but also is a function of the instantaneous operating condition of the engine. Varying the acceleration enrichment to increase the amount of enrichment during lean engine operations and decrease the amount of enrichment during rich engine operations will produce a system more closely related to the ideal.
One of the more advantageous types of closed loop integral controller systems in the prior art uses an O.sub.2 sensor for detecting rich or lean excursions of the air/fuel ratio by sensing the presence of oxygen in the exhaust gases. These systems usually operate at an average air/fuel ratio that is stoichiometric or slightly offset from that point. An acceleration enrichment feature sensitive to the instantaneous air/fuel ratio will assist in maintaining the emission levels in these systems.
Another of the more advantageous types of closed loop integral controllers is one which operates with a mixture so lean that the engine will just begin to run rough. The roughness threshold or average air/fuel ratio operating point for this system is set by the driveability criteria of the auto and acceleration enrichment for transients is necessitated to maintain this point. An acceleration enrichment feature sensitive to the instantaneous air/fuel ratio is therefore more important to this type of system because excursions for any length of time to the lean side of the threshold will be immediately felt by the operator as hesitations, roughness, or even stalls. Excursions on the rich side for any length of time will be defeating one of the primary purpose of the system, that of fuel economy.